Latest publication is now out in the Journal of Experimental Marine Biology and Ecology:
http://dx.doi.org/10.1016/j.jembe.2014.02.005
Working with Joe Pawlik, at UNCW, we examined the relationship between an obligate sponge-dwelling brittlestar and its host sponge. Using field and lab experiments, we conclude that the sponge has no apparent fitness gain from the brittlstar living and feeding on the surface of the sponge. The brittlestar, gains a surface to feed and refuge from predation as shown by Hendler 1984, and now we see can also consume larvae released by the host sponge.
Here’s a short video of larval predation in the lab. Check out the full paper for evidence of larval predation in the field as well. Based on this study, the brittlestar appears to vary between being a simple commensal of the sponge to a parasite consuming sponge larvae when available.